1. Field of the Invention
The invention relates to a heat sink having a base plate incorporating at least an evaporator portion of a heat pipe, and cooling fins which dissipate heat so that a cooling medium will condense in a condenser portion of the heat pipe.
2. Description of the Related Art
Components such as high power CPU's in a computer generate heat which must be removed so that the component will not overheat and fail. To this end, such components are provided with copper lids to which a heat sink is fixed using mounting hardware which engages the PCB or other substrate to which the component is mounted. The lids generally have a hot spot from 10 to 25 mm square generating 80–150 watts or more. Heat sinks generally include a base plate having a bottom surface which is fixed against the copper lid on the component, and a top surface having cooling fins which dissipate the heat to the surrounding air, which may be circulated.
High efficiency heat sinks include a heat pipe having at least a portion embedded in a base plate of the heat sink; “heat pipe” is a term which refers to a closed pipe containing a working fluid such as water which is present in two phases, liquid and gas. A portion of the heat pipe is placed over the hot spot and thereby serves as an evaporator, while a portion remote from the evaporator serves as a condenser which returns liquid to the evaporator. The evaporation and condensing cool the component efficiently until the heat load becomes so great that the working fluid evaporates faster than the condensing fluid can return to the evaporator, at which point the evaporator “dries out”. Once the evaporator dries out, the increase in temperature with increased power is orders of magnitude greater than before the dry out point was reached. The process of moving heat is essentially reduced to conduction by the thin walled tube.
Heat sinks with heat pipes entirely embedded in a base plate are best suited for short fin heights. The heat pipes are preferably pressed into channels in the bottom of the base plate and machined flush so that the heat pipes present a flat surface for good heat transfer from the hot spot on the component. A liquid cooled heat sink with a conduit formed in this way is disclosed in U.S. Pat. No. 5,829,516. Multiple heat pipes, for example configured as interlocking U's having adjacent inside arms over the hot spot, transfer heat to the condenser portions in the outside arms at a much higher rate than the material of the base plate. Single heat pipes with multiple evaporators are also known, e.g. in the shape of a heart having substantially parallel ends which serve as evaporators and an outside loop for the condenser, and a “figure 8” formed with two loops connected by an intermediate section extending between the ends.
Tower style heat sinks employ a base plate with channels which receive evaporator portions of “L” or “U” shaped heat pipes having upstanding condenser portions which carry the heat perpendicular to the base into a horizontal fin stack. This is especially effective for tall heat sinks (3″+). For shorter heat sinks the fins on the bend radii of the heat pipes provide a significant part of the fin area, and can adversely affect heat transfer to the fins. This results from the necessity of providing varying hole spacings in the fins, or slots which only contact one side of the heat pipe at the bend radii. This type of heat sink is also hard to design due to the conflict between locating the mounting hardware and optimal location of the heat pipes.
Heat sinks have also been designed with “U” shaped heat pipes having one arm serving as an evaporator embedded in the base plate, and another arm which loops into a vertical fin structure upstanding from the base plate. The main drawback is that the bend or bight of the U must be located outside of the fins, or placed within large notches in the fins. A heat sink of this type is disclosed by U.S. Pat. No. 6,189,601.